Signum Documentation
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SelectMany is a very simple operator, however, is sometimes hard to grasp for beginners. You can see more about how SelectMany is meant to work here.
On important difference of the implementation SelectMany in LINQ to Signum over Linq to SQL is that a big part of Linq to Sql code base is, in words of Matt Warren:
to reduce
CROSS APPLY's intoCROSS JOINin an opportunistic fashion.
We save ourselves from writing this code, instead we use CROSS APPLY any time a SelectMany appears.
So if you write a query like this
IQueryable<CommentEntity> result = Database.Query<BugEntity>().SelectMany(b => b.Comments);
//Or the equivalent using query expressions
IQueryable<CommentEntity> result = from b in Database.Query<BugEntity>()
from c in b.Comments
select c;SelectMany will always translated to CROSS APPLY.
SELECT s1.Text, s1.Date, cdn.Id, cdn.Ticks, cdn.Name, s1.idWriter_Customer, ddn.Id AS Id1, ddn.Ticks AS Ticks1, ddn.Name AS Name1, s1.idWriter_Developer, s1.HasValue
FROM BugEntity AS bdn
CROSS APPLY (
(SELECT bdnc.Text, bdnc.Date, bdnc.idWriter_Customer, bdnc.idWriter_Developer, bdnc.HasValue
FROM BugDNComments AS bdnc
WHERE (bdn.Id = bdnc.idParent))
) AS s1
LEFT OUTER JOIN CustomerEntity AS cdn
ON (s1.idWriter_Customer = cdn.Id)
LEFT OUTER JOIN DeveloperEntity AS ddn
ON (s1.idWriter_Developer = ddn.Id)If someone wants LINQ to Signum to work on other DBMS, like PostgreSQL, MySQL or Oracle, this will be a difficult part.
The second difference is not an implementation detail like the firts one:
We support OUTER APPLY by adding DefaultIfEmpty at the end of the selector lambda.
So a query like this:
IQueryable<CommentEntity> result = from b in Database.Query<BugEntity>()
from c in b.Comments.DefaultIfEmpty()
select c;Will return one null element for each BugsEntity that have no Comments, instead of just skipping it.
This is thanks to the use of OUTER APPLY instead of CROSS APPLY.
SELECT s1.Text, s1.Date, cdn.Id, cdn.Ticks, cdn.Name, s1.idWriter_Customer, ddn.Id AS Id1, ddn.Ticks AS Ticks1, ddn.Name AS Name1, s1.idWriter_Developer, s1.HasValue
FROM BugEntity AS bdn
OUTER APPLY (
(SELECT bdnc.Text, bdnc.Date, bdnc.idWriter_Customer, bdnc.idWriter_Developer, bdnc.HasValue
FROM BugDNComments AS bdnc
WHERE (bdn.Id = bdnc.idParent))
) AS s1
LEFT OUTER JOIN CustomerEntity AS cdn
ON (s1.idWriter_Customer = cdn.Id)
LEFT OUTER JOIN DeveloperEntity AS ddn
ON (s1.idWriter_Developer = ddn.Id)© Signum Software. All Rights Reserved.
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